In such a system it is known to employ, at a switching terminal of a central office, a switch marker controlling a line concentrator/distributor which on an outgoing call connects the line of the calling subscriber to an available trunk or other inter-office link and on an incoming call connects a designated link to a selected local subscriber line; see, for example, U.S. Pat. No. 3,328,534. The markers at the intercommunicating terminals are interconnected via a common signal path which carries the information relating to existing or desired connections between a given link and a local subscriber line. The line concentrator/distributor (simply referred to hereinafter as a line concentrator), inserted between n subscriber lines and m links where m is substantially smaller than n, comprises a multiplicity of switches whose selective closure thus allows up to m conversations to be carried out simultaneously. The marker, as a central component, communicates with the local lines and the inter-office links through the intermediary of peripheral units such as test circuits and couplers.
Since the establishment of a connection for the exchange of messages between the lines of the calling and called subscribers takes place independently of the inter-office line selected for a particular call, it behooves to check the continuity of the links before terminating this phase of operation, i.e. before releasing the centralized components involved and summoning the called party. Thus, if the link is found to be defective, or if the switches of the line concentrator at either end thereof do not properly couple that link to the corresponding local line, corrective measures will have to be taken, as by reoperating the connectors for another try, selecting a different link and/or emitting an alarm signal. The operation of a call-signal generator for the transmission of ringing current to the called subscriber station may also be made conditional upon a verification of continuity.
Such verification is particularly important in the case of certain subscribers, e.g. airlines or banks, which must have reliable telecommunication service available at all times.